Andromeda went on sale pretty soon after release and currently goes down to about $20-30 USD. Here are some threads discussing this topic, but most users will say that the game is worth getting at the discounted price.

Do I need to play the OT before playing Andromeda?

Technically, no. However, it is generally highly recommended you do so, as playing the OT will greatly enhance your understanding of MEA.

Because you need 2 squadmates for the battle sequences on Menae. If Garrus isn't alive (and because he's not replaced with another Turian) then Liara has to stay on Menae instead of going back to the Normandy and dealing with the EDI situation.

I was going to say that the administrator replaced her but I wasn't too sure, my first ME3 playthrough wasn't with an import and learned about Jack's position in Grissom afterwards and thought the student was always there.